Please attach some sample data or screenshot of your requirement or a packaged workbook. However, if you want to find out the difference then you have to create a calculated field in which you can subtract the formulas you used to find out the number of issues for both the condition.

I do not think that this is possible in tableau as the problem lies with the way the data is shaped. Having both the closed date and open date within the same record will cause problems if you want to compare both open and close events at the same time. You could either solve this by modifying the datasource so the data is placed in the proper shape. You can achieve the same with custom SQL. For reshaping check this thread & follow the steps of Joe Meko. http://community.tableau.com/thread/105620

Got it working with a roundabout process - because I cant change the SQL database we are using, create a tableau export and then use custom SQL on that as described in the thread you linked.

Now I have a working example of it I do come across another problem where if I make a rolling average to show the number of active issues over time and filter the date (for example, to show this year's issues), the rolling average resets to zero where I want it to continue.

I tried using aggregate measures and attributes to fix this, but Tableau doesnt like it because there are duplicate dates down the date column. Any ideas on this?